Difference of the Gas Density Histograms in and out of spiral arms in Milky Way Galaxy

"Gas Density Histogram (GDH)" is an observational counterpart of the probability density function (PDF) of the gas density of interstellar medium (ISM). We used ¹²CO data in (l, b) = (29°, 0) region from "FOREST unbiased galactic imaging survey with Nobeyama 45-m telescope (FUGIN)", which is a large coverage survey in three CO (1-0) lines. Using the kinetic distance, we estimated the volume density of the voxel from the observed column density. The resultant GDHs of the inter-arm regions show lognormal or lognormal-like, but those in the spiral arm regions show flat-top shape.